The present invention relates generally to apparatus for controlling the flow of a gas.
Industrial processes such as those used in the fabrication of semiconductor wafers in furnaces require a continuous flow of a process gas mixture to the furnace process site. A typical semiconductor fabrication process requires initial use of a particular mixture of gases, both reactive and inert, followed by a sequence of different gas mixtures as the process proceeds The rate of flow of the gases, their initiation and their shut off are under the control of one or more mass flow controllers.
The gas mixture proceeds from a mixing station or manifold through a conduit system which includes conventional tubing, hoses, control valves, the mass flow controllers and like components.
On completion of a processing step, closure of the associated control valve will halt the flow of process gas to the process site. However, remnants of the gas will remain in the system. Since the next process step may require use of a different mixture of gases, remnants of the first process gas mixture must be purged or expelled from the conduit system to avoid contaminating the new process gas. In addition, it is desirable to expel process gas remnants promptly after the end of all processing to minimize the harmful effects of any corrosive or reactive component gases remaining in the conduit system. Preferably, such purging should be done under automatic control. Although various ways of expelling remnants of process gases have been proposed, none has been entirely satisfactory.
Some components of process gas mixtures are obtained by vaporizing a liquid. Various means for vaporizing such a liquid and controlling flow of the resulting vapor have been proposed, but each suffers certain drawbacks, particularly complexity and difficulty of controlling the rate at which the vapor enters the associated control valve, especially if the process is being carried out at a pressure close to atmospheric.
It will be apparent from the foregoing that there is a need for gas flow control apparatus which provides a simple, automatically controllable means to purge process gas from as much of the conduit means as possible, and to minimize the effects of any such gas remaining in the conduit system after process gas shut off, and which also provides a simple means for vaporizing a liquid and controlling the flow rate of the vapor into the process gas mixture.